Compensating for off-current in a memory

ABSTRACT

A memory cell is accessed by determining an off-current of a set of memory cells, accessing a memory cell of the set of memory cells during an access period, and compensating for the off-current of the set of memory cells.

BACKGROUND

1. Technical Field

The present subject matter relates to semiconductor memory, and moreparticularly, to compensating for an off-current of the memory.

2. Background Art

Many types of semiconductor memory are known in the art. One type ofmemory is flash memory which stores charge in a charge storage region ofa memory cell. The voltage threshold of the metal-oxide-semiconductorfield-effect transistor (MOSFET) based flash cell can be changed bychanging the amount of charge stored in the charge storage region of thecell, and the voltage threshold can be used to indicate a value that isstored in the flash cell. By providing a voltage across the flash cellthat is between the voltage thresholds of the two different states ofthe flash cell, the state of the flash cell can be determined bymeasuring current flowing through the flash cell. A flash cell has amuch higher on-current than off-current.

Another type of memory is phase change memory (PCM). PCMs utilize aphase change material having a non-conductive amorphous state and aconductive crystalline state. A PCM cell may be put into one state orthe other to indicate a stored value. By providing a potential acrossthe PCM cell, the state of the PCM cell can be determined by measuringcurrent flowing through the PCM cell. A PCM cell has a much higheron-current than off-current.

One architecture in common use for memories is a NAND architecture. In aNAND architecture, two or more memory cells are coupled together into astring, with the individual cell control lines coupled to word lines. ANAND string may be coupled to a bit line at one end of the NAND string.A bit of a NAND string may be read by addressing one cell of the stringand turning on all the other cells in the string. If the addressedmemory cell is on, the on-current will flow through the string, but ifthe addressed memory cell is off, only an off-current, which is muchlower than the on-current will flow.

Some memory devices may create stacks of memory cells in athree-dimensional array. A stack of memory cells may include any numberof memory cells. In some cases, the cells of a stack may be coupled intoa NAND string but in other cases, multiple layers of control lines maybe used to address cells of layer individually.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute partof the specification, illustrate various embodiments. Together with thegeneral description, the drawings serve to explain various principles.In the drawings:

FIG. 1A is a block diagram of an embodiment of a memory that compensatesfor off-current;

FIG. 1B is a more detailed block diagram of an embodiment of a memorythat compensates for off-current;

FIG. 2A is a schematic of an embodiment of a memory that compensates foroff-current;

FIG. 2B is a cross-sectional view of stacked NAND memory cells;

FIG. 3 is a schematic of an alternative embodiment of a memory thatcompensates for off-current;

FIG. 4A-4C are flow charts of various aspects of embodiments of methodsfor compensating for off-current of a memory; and

FIG. 5 is a block diagram of an embodiment of an electronic system withmemory that compensates for off-current.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth by way of examples in order to provide a thorough understanding ofthe relevant teachings. However, it should be apparent to those skilledin the art that the present teachings may be practiced without suchdetails. In other instances, well known methods, procedures andcomponents have been described at a relatively high-level, withoutdetail, in order to avoid unnecessarily obscuring aspects of the presentconcepts. A number of descriptive terms and phrases are used indescribing the various embodiments of this disclosure. These descriptiveterms and phrases are used to convey a generally agreed upon meaning tothose skilled in the art unless a different definition is given in thisspecification. Reference now is made in detail to the examplesillustrated in the accompanying drawings and discussed below.

FIG. 1A is a block diagram of an embodiment of a memory 100 thatcompensates for off-current. The memory 100 may include two or morememory cells 110, which may be a large number of memory cells, such asmillions or even billions of memory cells or more, depending on theembodiment. The memory cells 110 may include any type of memorytechnology that has different current flow through a memory celldepending on the state of the memory cell, such as, but not limited tofloating gate flash, charge trap flash (CTF), nanowire based memory,ferro-electric transistor random access memory (FeTRAM), resistiverandom access memory (RRAM), phase-change memory (PCM), and PCM withswitch (PCMS). The memory cells 110 may use any logical architecture,including NAND, NOR, or other logical architecture, and may use anyphysical embodiment, such as a traditional two dimensional memory array,multiple layers of two-dimensional arrays built on top of each other,vertical stacks of memory cells organized as a three dimensional array,or other physical embodiments. The memory cells 110 may be organized tocouple sets of memory cells to sense nodes, and in many embodiments, anindividual memory cell may be coupled to a single sense node, althoughother embodiments may allow for an individual memory cell to beselectively coupled to more than one sense node.

The memory cells 110 may be coupled to I_(OFF) compensation circuitry120 which may be capable of determining an off-current (I_(OFF)) of thememory cells 110. The I_(OFF) compensation circuitry 120 may be coupledto the sense nodes and, in embodiments, may be capable to determine acombined I_(OFF) of the set of memory cells coupled to an individualsense node and/or compensate for the combined I_(OFF) of the set ofmemory cells coupled to an individual sense node. In some embodimentsI_(OFF) may said to be determined by setting a maximum level of currentin the I_(OFF) compensation circuitry 120 based on I_(OFF).

Sense circuitry 130 may be coupled to sense nodes of the memory cells110. The sense circuitry 130 may determine a state of an accessed memorycell in the memory cells 110 during an access period. In someembodiments, the sense circuitry 130 may provide a load current to asense node during the access period and detect a voltage level on thesense node to determine the state of the accessed memory cell. In otherembodiments, the sense circuitry 130 may not include a current source,and may simply measure, or detect, an electrical parameter such asvoltage, of the sense node to determine the state of the memory cell.The sense circuitry 130 may provide an output 104 that indicates thedetected state of the accessed memory cell.

The memory 100 may include control circuitry 140 to receive an access ofthe memory cells 110 through an interface 102. In some embodiments, theinterface 102 and the output 104 may utilize at least some of the samephysical conductors. The control circuitry 140 may include a finitestate machine, a micro-sequencer with microcode to execute on themicro-sequencer, a processor with instructions to execute on theprocessor, or any combination of such mechanisms or other mechanisms forcontrolling various lines in electronic circuitry. The control circuitry140 may decode the access and determine which memory cell of the memorycells 110 is to be accessed. The control circuitry 140 may initiate anaccess period in response to the access and may communicate with thememory cells 110, I_(OFF) compensation circuitry 120, and the sensecircuitry 130 to determine the state of the accessed memory cell withcompensation for the combined I_(OFF) of the memory cells coupled to thesame sense node as the accessed memory cell. The control circuitry 140may also initiate a sample period to determine I_(OFF). The sampleperiod may occur in response to an access, a period of time since thelast sample period, a change of temperature of the memory 100, or anyother stimulus, depending on the embodiment.

In some embodiments, the set of memory cells coupled to an individualsense node may include a large number of memory cells, such as millionsof memory cells or more. While the off-current of a single memory cellmay be negligible compared to the current that is able to flow throughthe single memory cell in the on state, I_(ON), the combined I_(OFF) forthe set of memory cells coupled to a single sense node may be largeenough to make it difficult to reliably detect the difference betweenthe combined I_(OFF) of the set of memory cells coupled to a singlesense node and I_(ON). The I_(OFF) compensation circuitry 120 maycompensate for the combined I_(OFF) of the set of memory cells to makedetermining the state of the accessed memory cell more reliable. Bycompensating for the combined I_(OFF) of the memory cells coupled to asense node, it may be possible reliably detect the state of a singlememory cell with many more memory cells coupled to a single sense nodethan would be possible without I_(OFF) compensation.

FIG. 1B is a more detailed block diagram of an embodiment of a memory101 that compensates for off-current (I_(OFF)). The memory 101 mayinclude memory cells 111 that are coupled to a sense node 115. Sensecircuitry such as a sense amplifier 131 may be coupled to the sense node115 and be able to sense a voltage level of the sense node 115 so thatthe sense amplifier 131 may communicate the state of an accessed memorycell to an external or internal output 105. Compensation circuitry 121may include reference current circuitry 123 and compensation currentcircuitry 122 coupled to the sense node 115.

The control circuitry 141 may initiate a sample period to determine acombined I_(OFF) current of the memory cells 111 that are coupled to thesense node 115. During the sample period, the control circuitry 141 maydeselect the memory cells 111 and may enable the reference currentcircuitry 123 to provide a reference current at the sense node 115. Thereference current may be less than an on-current (I_(ON)) of a singlememory cell of the memory cells 111. The value of the reference currentmay be determined at design time, or may be based on measurements takenof I_(ON) during operation of the memory 101. The control circuitry 141may also set the compensation current circuitry 122 to provide up to amaximum value of compensation current during the sample period. Thecompensation current circuitry 122 may continue to provide up to the setmaximum value of compensation current at the sense node 115 after thesample period is ended, such as during the access period. The maximumvalue of the compensation current may be based on the combined I_(OFF)of the memory cells 111 and the reference current. To satisfy Kirchoff'scurrent law, the sum of the currents flowing into the sense node isequal to zero. So if the memory cells 111 are deselected so that I_(OFF)current is flowing through the memory cells 111, and if the currentflowing into the sense amplifier 131 is negligible, the maximum value ofthe compensation current, plus a value of the reference current, plus avalue of I_(OFF), is about equal to zero, with the currents beingdefined as positive flowing into the sense node 115.

The control circuitry 141 may initiate the access period in response tothe access of the memory received through the control input 103. Thecontrol circuitry 141 may select a memory cell of the memory cells 111and disable the reference current circuitry 123 during the accessperiod. The control lines used by the control circuitry 141 may varyaccording to the embodiment, but may include bit lines and word lines tothe memory cells 111. The control circuitry 141 may enable thecompensation current circuitry 122 to provide a compensation current atthe sense node 115 during the access period. The maximum value of thecompensation current may have been previously set during a sampleperiod. The control circuitry 141 may also enable the sense amplifier131 to determine a state of the accessed memory cell by sensing avoltage level of the sense node 115. Because the value of the referencecurrent provided by the reference current circuitry 123 during thesample period was less than I_(ON) of an accessed memory cell, if theaccessed memory cell is ‘OFF,’ the maximum current provided by thecompensation current circuitry 122 is more than I_(OFF), causing thevoltage of the sense node 115 to rise and the output 105 to be a ‘0’. Ifthe accessed memory cell is ‘ON,’ I_(ON), which is greater than thereference current, is greater than the maximum compensation currentprovided by the compensation current circuitry 122, allowing the maximumcompensation current to flow through the accessed memory cell, so thesense node 115 is at a low voltage level and the output 105 is a ‘1’.

In the memory 101, the following equations describe the setting amaximum value (I_(MAXCOMP)) of the compensation current (I_(COMP)) fromthe compensation current circuitry 122 during a sample period based onthe reference current (I_(REF)) and the off-current (I_(OFF)) with _(I)_(COMP) defined as a positive value for current flowing into the sensenode 115, and I_(REF), I_(ON) and I_(OFF) defined as positive values forcurrent flowing out of the sense node 115.

I _(MAXCOMP) −I _(REF) −I _(OFF)=0 so I _(MAXCOMP) =I _(REF) +I _(OFF)

Where: I_(COMP)≦I_(MAXCOMP) and I_(REF)<I_(ON)

If a relatively large number of memory cells are coupled to the sensenode, having one memory cell ‘ON’ may not have a significant effect onthe value of the current flowing through the remaining memory cells thatare ‘OFF’, so the amount of current flowing through the remaining memorycells remains essentially I_(OFF). The current through the accessedmemory cell in an ‘ON’ state is I_(MEM) which is defined as a positivevalue out of the sense node 115. So the following equations representthe current during the access period if the accessed memory cell is inthe ‘ON’ state:

I _(COMP) −I _(MEM) −I _(OFF)=0 so I _(COMP) =I _(MEM) +I _(OFF) and

I _(COMP) ≦I _(MAXCOMP) =I _(REF) +I _(OFF) <I _(ON) +I _(OFF) so:

I _(MEM) +I _(OFF) <I _(ON) +I _(OFF) and

I_(MEM)<I_(ON)

The equations above show that the current flowing through the accessedmemory cell is less than the current that could flow through the memorycell in the ‘ON’ state, so the voltage at the sense node 115 is aboutequal to the voltage across the ‘ON’ memory cell, which is a relativelylow voltage.

If the accessed memory cell is in the ‘OFF’ state, the followingequations apply during the access period:

I _(COMP) −I _(OFF)=0 so I _(COMP) =I _(OFF) and I _(MAXCOMP) =I _(REF)+I _(OFF) so:

I_(COMP)<I_(MAXCOMP)

The equations above show that the actual compensation current, if theaccessed memory cell is ‘OFF,’ is less than the maximum compensationcurrent set during the sample period. This allows the compensationcurrent to charge the capacitance of the sense node 115 and causes thevoltage at the sense node 115 to rise to something close to the supplyvoltage of the compensation current source 122.

In some embodiments, the control circuitry 14l may receive a secondmemory access and initiate a second access period without initiatinganother sample period to re-set the maximum compensation current basedon I_(OFF). I_(OFF) may not change much in the time between memoryaccesses so the maximum compensation current may not be changed forevery access. So the control circuitry 141 may initiate a second accessperiod, disable the reference current circuitry 123, and enable thecompensation current circuitry 122 to provide up to the maximum value ofcompensation current at the sense node 115 during the second accessperiod. The control circuitry 141 may also enable the sense circuitry131 to determine a state of another memory cell in the memory cells 111by sensing a voltage level of the sense node 115.

A second sample period may be initiated by the control circuitry 141 forvarious reasons in embodiments. In at least one embodiment, atemperature of the memory cells 111 may be monitored and a sample periodinitiated in response to a change in the temperature of the memory cells111 because I_(OFF) may be dependent on temperature for some memorytechnologies. In some embodiments, a sample period may be initiated inresponse to a memory access, before the access period is initiated. Insome embodiments, a sample period may be initiated after a predeterminedperiod of time has elapsed since the previous sample period or inresponse to a memory access if the predetermined period of time haselapsed since the last sample period. In some embodiments, thepredetermined period of time is greater than about 1 microsecond (μs),although other embodiments may use shorter or longer predeterminedperiods. Other embodiments may initiate a sample period based on otherevents.

During a sample period, the control circuitry 141 may deselect thememory cells 111, enable the reference current circuitry 123 to providethe reference current at the sense node 115, and re-set the compensationcurrent circuitry 122 to provide up to a new maximum value ofcompensation current at the sense node 115 during an access period. Thenew maximum value of the compensation current is based on a combinedoff-current of the memory cells 111 during the sample period, and thereference current.

FIG. 2A is a schematic of an embodiment of a memory 200 that compensatesfor off-current. In some embodiments the memory 200 may include two ormore memory cells 201 that are organized into two or more NAND stringsof memory cells. The memory 200 shown includes a set of memory cells 201coupled to a sense node 255. In the embodiment shown, the memory cells201 are organized in a NAND architecture. A first NAND string 210 iscoupled to a bit line 250 by source select gate 213 and to a drain line219 by drain select gate 214. A source select line 217 may control thesource select gate 213 and a drain select line 218 may control the drainselect gate 214. The first NAND string 210 includes a first flash cell211 and a second flash cell 212. A first word line 215 is coupled to thecontrol gate of the first flash cell 211 and a second word line 216 iscoupled to the control gate of the second flash cells 212. A second NANDstring 220, which also includes two flash cells, is coupled to the bitline 250 and is controlled by a variety of control lines 225-228. Athird NAND string 230, which also includes two flash cells, is coupledto the bit line 250 and is controlled by a variety of control lines235-238. Control lines 215-218, 225-228, 235-238 may be driven fromcontrol circuitry (not shown in FIG. 2A). Other embodiments may includeNAND strings with any number of memory cells. Some embodiments may haveNAND strings with a number of memory cells that is a power of 2 such as8, 16, 32, 64, or larger powers of 2. Embodiments may have any number ofNAND strings coupled to the bit line 250.

In at least one embodiment, the memory cells 201 may be organized as athree-dimensional array of NAND flash with individual word lines coupledto rows of individual cells on a single layer, and bit lines coupled tocolumns of NAND strings at the ends of the NAND strings. FIG. 2B is across-sectional view of stacked NAND memory cells 201B. A substrate 202Bmay be a silicon wafer or other suitable base for constructing thevertical NAND stack. A drain line 219A may be coupled to a semiconductorpillar 205B. The semiconductor pillar 205B may be any suitablesemiconductor, but may be made of polysilicon in some embodiments. Adrain select gate 214B, using a portion of the pillar 205B as thechannel, may be formed to allow the drain line 219A to be isolated from,or coupled to, the rest of the pillar 205B. A NAND string 210B may beformed on top of the drain select gate 214B that includes a first flashcell 211B and a second flash cell 212B that utilize the semiconductorpillar 205B as their channels as well as to couple the two flash cells211B, 212B into a NAND string. A source select gate 213B may be formedon top of the NAND string 210B to couple the NAND string 210B to the bitline 250B. Various other three-dimensional structures may be used inother embodiments. So the two or more memory cells 201 may include oneor more stacks of memory cells.

An isolation transistor 251 may couple the bit line 250 to anintermediate node 252 in some embodiments. In some embodiments,additional isolation transistors may couple additional bit lines to theintermediate node 252 which may couple additional memory cells to theintermediate node. A control voltage V1 may be driven from the controlcircuitry. Reference current circuitry 260, which may be a singletransistor in some embodiments although other embodiments may utilizedifferent circuitry, may be coupled between the intermediate node 252and ground. The current level of the reference current circuitry 260 maybe determined by the voltage level V2 which may be driven by the controlcircuitry. A clamping transistor 253, driven by voltage V3 from thecontrol circuitry, may be used in some embodiments to couple theintermediate node 252, and therefore the memory cells 201, to the sensenode 255, and to limit a voltage level driven from the sense node 255 tothe intermediate node 252 and then on to the memory cells 201. A senseamplifier 280 may also be coupled to the sense node 255 to determine avoltage level of the sense node 255. The output 281 of the senseamplifier 280 may be used to indicate a state of a selected memory cell.

Compensation current circuitry 270 may be coupled to the sense node 255.Various embodiments may utilize different circuits for the compensationcurrent circuitry 270, but in the embodiment shown, a p-channel fieldeffect transistor (pFET) 273 may be used to control a current flowingfrom a positive supply voltage to the sense node 255. Another pFET 272may be used as a capacitor, with both source and drain coupled to thepositive supply voltage and the control gate of the pFET 272 coupled tothe control gate of the pFET 273. Another pFET 271 may be used to setthe voltage level at the control gate of the pFET 273. Control circuitrymay drive the voltage level V4 to the control gate of the pFET 271. IfV4 is grounded, pFET 271 is turned on and the voltage level of thecontrol gate of pFET 273 may vary. If V4 is at a voltage level of thepositive supply voltage or higher, pFET 271 is turned off and the pFET272, acting as a capacitor, may maintain the voltage level at thecontrol gate of the pFET 273, thereby limiting the current of thecompensation current circuitry 270.

During a sample period, the control circuitry may drive the controllines 215-218, 225-228, 235-238 of the memory cells 201 to deselect thememory cells 201. The voltage levels of the control lines 215-218,225-228, 235-238 to deselect the memory cells 201 may vary according tothe embodiment. With the memory cells 201 deselected, only an I_(OFF)current may be flowing through the memory cells 201. The controlcircuitry may also drive V1 to turn on the isolation transistor 251 andV3 to turn on the clamping transistor 253 to allow I_(OFF) to be appliedat the sense node 255. The voltage level V2 from the control circuitrymay set an amount of reference current flowing from the sense node 255.The voltage level V2 may be chosen at design time to set the referencecurrent level to be less than an expected on-current from a selectedmemory cell for the set of memory cells 201. In one embodiment utilizingNAND flash cells, an expected on-current might be a minimum of 100nanoamperes (nA), so the reference current might be set to about 50 nA.Other embodiments may have different expected on-current and associatedreference current levels, such as a memory with a minimum expectedon-current of 1 milliamp (mA) and a reference current of about 500micro-amperes (μA), or a memory with a minimum expected on-current of 10nA and a reference current of about 5 nA, although other embodimentsthat are any value of current and may be higher or lower than theexamples discussed. The control circuitry may then drive V4 to ground toturn on pFET 271 and set the compensation current circuitry 270 to havea maximum current level of about 500 μA+I_(OFF). At the end of thesample period, V4 may be driven back to a high level to turn off pFET271 so that the maximum current of the compensation current circuitry270 remains set to about 500 μA+I_(OFF), and then drive V2 to ground toturn off the reference current circuitry 260.

During an access period, the control circuitry may drive the controllines 215-218, 225-228, 235-238 of the memory cells 201 to select amemory cell of the memory cells 201. As an example, to select the firstmemory cell 211 of the NAND string 210, the control lines 225-228 of thesecond NAND string 220 and the control lines 235-238 of the third NANDstring 230 may be driven so that the second NAND string 220 and thethird NAND string 230 are deselected such as driving the control lines225-228, 235-238 to ground. The control circuitry may drive the drainline 219 to a low level and turn on the drain select gate 214 usingdrain select line 218 and the source select gate 213 using the sourceselect line 217 while providing a voltage on the second word line 216that is high enough to turn on the second memory cell 212 regardless ofits state. The first word line 215 may then be driven to a voltage levelthat is between the voltage threshold to turn on the first memory cell211 if the first memory cell 211 is ‘ON,’ and the voltage threshold toturn on the first memory cell 211 if the first memory cell 211 is ‘OFF’.The control circuitry may also drive V1 to turn on the isolationtransistor 251 and V3 to turn on the clamping transistor 253. With V2driven to ground to keep the reference current circuitry 260 off, and V4driven to ground to keep the maximum current of the compensationscurrent circuitry 270 unchanged, the compensation current may flow fromthe compensation current circuitry 270 through the clamping transistor253 and the isolation transistor 251 into the memory cells 201.Compensation current approximately equal to I_(OFF) may flow through thedeselected memory cells, with the remainder available to flow throughthe first NAND string 210 that contains the selected memory cell 211.

Consistent with the example above, say that the on-current flowingthrough the NAND string 210 can be at least 100 nA if the first memorycell 211 is ‘ON,’ which is greater than the value of the referencecurrent during the sample phase of about 50 nA. So if the state of thefirst memory cell 211 is ‘ON,’ the maximum value of the compensationcurrent flows from the compensation current circuitry 270 withcompensation current approximately equal to I_(OFF) flowing through thedeselected memory cells, and about 50 nA flowing through the first NANDstring 210. Because the current flowing through the selected memory cell211 is less than a maximum on-current of the memory cell 211, the sensenode 255 is at a low voltage level which may be detected by the senseamplifier 280.

If the state of the first memory cell 211 is ‘OFF,’ the compensationcurrent flowing from the compensation current circuitry 270 during theaccess period is only about I_(OFF), which is less than the maximumcompensation current set during the sample period. Because of this, thecompensation current circuitry 270 may bring the voltage level of thesense node 255 high, which may be detected by the sense amplifier 280.

FIG. 3 is a schematic of an alternative embodiment of a memory 300 thatcompensates for off-current. The memory 300 uses phase change memorywith switch (PCMS) cells 301 in a NOR architecture. An exemplary PCMScell 311 includes phase change material 308 and a switch 306 coupledbetween a word line 305 and a bit line 350. Additional PCMS cells312-319 may be coupled to the bit line 350. Any number of PCMS cells maybe coupled to the bit line 350 in various embodiments. Some embodimentsmay include an isolation transistor 351, controlled by a voltage levelV5, to couple the bit line 350 to an intermediate node 352. Someembodiments may have additional isolation transistors that may coupleadditional bit lines and their associated memory cells to theintermediate node 352. Reference current circuitry 360 may be coupled tothe intermediate node 352. Some embodiments may include a clampingtransistor 353, controlled by voltage level V6 to couple theintermediate node 352 to the sense node 355, although other embodimentsmay not include a clamping transistor 353. A sense amplifier 380 may becoupled to the sense node 355 to detect a voltage level of the sensenode 355 and provide an output 381.

Compensation current circuitry 370 and load current circuitry 390 may becoupled to the sense node 355. The load current circuitry may include apFET controlled by a voltage level V9 from the control circuitry withthe voltage of V9 determining a maximum load current provided by theload current circuitry 390 from the positive voltage source to the sensenode 355. The compensation current circuitry 370 may include a fieldeffect transistor (FET) 373 coupled from the sense node 355 to groundwith a control gate driven from the FET 371. Another FET 372 may act asa capacitor to hold the control gate of the FET 373 at a constant valueif the FET 371 is off.

During a sample period, the control circuitry may deselect the memorycells 301 by driving the word lines, such as word line 305, high. Thecontrol circuitry may drive V5 and V6 to turn on the isolationtransistor 351 and clamping transistor 353, and drive V7 with a voltageto allow up to a predetermined reference current value to flow throughthe reference current circuitry 360. The control circuitry may alsodrive V9 with a voltage to allow up to a predetermined load current toflow through the load current circuitry 390 to the sense node 355. Thecontrol circuitry may then drive V8 high to turn on FET 371 so that themaximum amount of the compensation current flowing through thecompensation current circuitry 370 is set based on I_(OFF). Once themaximum amount of the compensation current is set, the FET 371 may beturned off by the control circuitry driving V8 to ground. With thememory cells 301 deselected, the current flowing through the isolationtransistor 351 and through the memory cells 301 is an off-current,I_(OFF). In memory 300 the maximum value of the compensation current,plus the maximum value of the load current, plus a value of thereference current, plus a value of I_(OFF) of the memory cells 201, isabout equal to zero, by Kirchoff's current law applied to the sense node355. So the maximum value of the compensation current set for thecompensation current circuitry 370 during the sample period is dependenton I_(OFF) and may be shown by the equations:

I _(LOAD) −I _(COMPMAX) −I _(REF) −I _(OFF)=0 so I _(COMPMAX) =I _(LOAD)−I _(REF) −I _(OFF)

During an access period, the control circuitry may drive the word line305 low to select the memory cell 311. The word lines associated withthe other memory cells 312-319 may be driven high to deselect the othermemory cells 312-319. The control circuitry may turn on the isolationtransistor 351 using V5 and the clamping transistor 353 using V6, anddisable the reference current circuitry 360 by driving V7 to ground. Theload current circuitry 390 is enabled with V9 to drive up to the maxload current and the compensation current circuitry 370 continues tosink up to the maximum set value of the compensation current. So if thecurrent through the selected memory cell 311 is I_(MEM) and the numberof memory cells coupled to the bit line 350 is large so that the currentthrough the unselected memory cells is still about I_(OFF), the equationfor Kirchoff's current law at the sense node 355 is:

I _(LOAD) −I _(COMP) −I _(MEM) −I _(OFF)=0 so I _(MEM) =I _(LOAD) −I_(COMP) −I _(OFF)

with I_(COMP)≦I_(COMPAX):

I _(MEM) ≦I _(LOAD)−(I _(LOAD) −I _(REF) −I _(OFF))−I _(OFF) or

I_(MEM)≦I_(REF)

Because I_(REF) was chosen to be less than an expected maximumon-current of a memory cell, if the selected memory cell 311 is ‘ON,’I_(MEM)=I_(REF) and the voltage at the sense node 355 is a low voltageabout equal to the ‘ON’ voltage of the selected memory cell 311, whichmay be detected by the sense amplifier 380. If the selected memory cell311 is ‘OFF,’ very little current flows through the selected memory cell311 so I_(MEM)<<I_(REF) and the voltage at the sense node 355 is a highvoltage because the compensation current circuitry 370 limits the amountof current that the load current circuitry 390 is able to source,causing the load current circuitry to drive the sense node 355 to avoltage near the positive supply voltage. The high voltage at the sensenode 355 may be detected by the sense amplifier 380.

While the compensation current circuitry 270 of FIG. 2A may includefewer transistors than the combined load current circuitry 390 and thecompensation current circuitry 370 of FIG. 3, some embodiments may usethe circuitry shown in FIG. 3. The circuitry shown in FIG. 3 may be ableto provide for a wider voltage range at the sense node 355 thancompensation current circuitry 270, which may be useful in some deviceswith low voltage power supplies.

FIG. 4A-4C are flow charts 400, 410, 420 of various aspects ofembodiments of methods for compensating for off-current of a memory.Flow chart 400 may begin with an access of memory at block 401. Theaccess of memory 401 can be any type of access but in some embodimentsthe access may be a read command, a program command, or an erasecommand. The flow chart 400 may continue in response to the access atblock 402 by determining an off-current (I_(OFF)) of a set of memorycells that are coupled to a sense node. Depending on the embodiment,I_(OFF) may have already been determined before the memory is accessedat block 401. Block 403 may include accessing a memory cell of the setof memory cells during an access period and at block 404 the flow chart400 continues with compensating for the off-current of the set of memorycells to determine a state of the accessed memory cell. If the accesswas a read command, the state of the memory cell may be provided inresponse to the read command. If the access was a program command or anerase command, the state of the memory cell may be checked to ensurethat the command was completed successfully. The flow chart 400 may waitfor the next memory access at block 405, ending the flow chart 400.

In at least one embodiment, the determining of the off-current in block402 may involve storing digital information regarding measurements ofthe off-current during a sample period. This may involve an analog todigital conversion of the voltage at the sense node with the set ofmemory cells deselected at block 402. Then a digital value based on ameasurement of current through the memory cell during the access periodmay be captured as a part of block 403. This may involve a second analogto digital conversion of the voltage at the sense node while accessing amemory cell at block 403. The off-current may then be compensated bysubtracting at least some of the digital information from the digitalvalue to determine a state of the memory cell at block 404.

In other embodiments, the determining of the off-current may involvesetting a maximum compensation current based on the off-current at block402. The accessing of the memory cell may include sensing a voltagelevel at the sense node to determine a state of the memory cell at block403. The compensating for the off-current may include providing acompensation current at the sense node during the access period at block404. The maximum value of the compensation current was set based on theoff-current at block 402.

In some embodiments, blocks 403 and 404 may be repeated withoutrepeating block 402. So the method may include accessing another memorycell of the set of memory cells during a second access period, andcompensating for the off-current during the second access period withoutre-determining the off-current. In some embodiments, it may bedetermined whether or not to include block 402 in response to a memoryaccess based on whether a predetermined period of time has elapses sincethe last determination of I_(OFF) of the set of memory cells. So someembodiments may include re-determining the off-current if more than apredetermined period of time has passed since the determining of theoff-current, accessing another memory cell of the set of memory cellsduring a second access period after the re-determining of theoff-current, and compensating for the re-determined off-current duringthe second access period. In some embodiments, the predetermined periodof time may be greater than about 1 μs.

FIG. 4B shows a flow chart 410 of a method to determine the off-currentof the set of memory cells, which might be used for some embodiments ofblock 402 of flow chart 400. Control circuitry may initiate a sampleperiod at block 411 to start determining the off-current. At least onesample period may occur before an access period. A sample period may beinitiated in response to a change of temperature of the set of memorycells in some embodiments. In other embodiments, the sample period maybe initiated before an access if the predetermined period of time haspassed since the last sample period as described above. The flow chart410 may continue by deselecting the set of memory cells coupled to thesense node at block 412. The control circuitry may provide a referencecurrent at the sense node during the sample period at block 413. Thevalue of the reference current may be less than an expected on-currentof a memory cell. At block 414 the compensation current circuitry may beset to provide up to the maximum value of the compensation current basedon I_(OFF). The compensation current circuitry may provide up to maximumvalue of compensation current to the sense node after the end of thesample period, such as during the access period. So the maximum value ofthe compensation current, plus a value of the reference current, plus avalue of the off-current, is about equal to zero, in some embodiments.In other embodiments, a load current may be provided at the sense nodeduring the sample period, so that the maximum value of the compensationcurrent, plus a maximum value of the load current, plus a value of thereference current, plus a value of the off-current, is about equal tozero.

FIG. 4C shows a flow chart 420 of a method to access a memory cell withcompensation for I_(OFF) such as might be done for embodiments of flowchart 400 in block 403 and block 404. The flow chart 420 may respond toan access of the memory by initiating an access period at block 421. Thecontrol circuitry may select a memory cell at block 422, and disable asource of reference current at block 423. In some embodiments, themethod may include providing a load current to the sense node at block424, although some embodiments may not include a load current. At block425, the compensation current may be provided at the sense node. Amaximum value of the compensation current may have been previously setduring a sample period. The voltage level at the sense node may besensed at block 426 to determine a state of the accessed memory cell.The next memory access may be waited on at block 427.

In various embodiments of the methods described by the flow charts 400,410, 420, the set of memory cells may include one or more NAND stringsof memory cells and/or one or more stacks of memory cells. In someembodiments, channels of the set of memory cells may include polysiliconand in some embodiments the set of memory cells may include phase changematerial.

FIG. 5 is a block diagram of an embodiment of an electronic system 500with memory 510 that compensates for off-current. Supervisory circuitry501 is coupled to the memory device 510 with control/address lines 503and data lines 504. In some embodiments, data and control may utilizethe same lines. The supervisory circuitry 501 may be a processor,microprocessor, microcontroller, finite state machine, or some othertype of controlling circuitry. The supervisory circuitry 501 may executeinstructions of a program in some embodiments. In some embodiments, thesupervisory circuitry 501 may be integrated in the same package or evenon the same die as the memory device 510. In some embodiments, thesupervisory circuitry 501 may be integrated with the control circuitry511, allowing some of the same circuitry to be used for both functions.The supervisory circuitry 501 may have external memory, such as randomaccess memory (RAM) and read only memory (ROM), used for program storageand intermediate data or it may have internal RAM or ROM. In someembodiments, the processor may use the memory device 510 for program ordata storage. A program running on the supervisory circuitry 501 mayimplement many different functions including, but not limited to, anoperating system, a file system, memory block remapping, and errormanagement.

In some embodiments an external connection 502 is provided. The externalconnection 502 is coupled to input/output (I/O) circuitry 505 which maythen be coupled to the supervisory circuitry 501 and allows thesupervisory circuitry 501 to communicate to external devices. In someembodiments, the I/O circuitry 505 may be integrated with thesupervisory circuitry 501 so that the external connection 502 isdirectly coupled to the supervisory circuitry 501. If the electronicsystem 500 is a storage system, the external connection 502 may be usedto provide an external device with non-volatile storage. The electronicsystem 500 may be a solid-state drive (SSD), a USB thumb drive, a securedigital card (SD Card), or any other type of storage system. Theexternal connection 502 may be used to connect to a computer or otherintelligent device such as a cell phone or digital camera using astandard or proprietary communication protocol. Examples of computercommunication protocols that the external connection may be compatiblewith include, but are not limited to, any version of the followingprotocols: Universal Serial Bus (USB), Serial Advanced TechnologyAttachment (SATA), Small Computer System Interconnect (SCSI), FibreChannel, Parallel Advanced Technology Attachment (PATA), IntegratedDrive Electronics (IDE), Ethernet, IEEE-1394, Secure Digital Cardinterface (SD Card), Compact Flash interface, Memory Stick interface,Peripheral Component Interconnect (PCI) or PCI Express (PCI-e).

If the electronic system 500 is a computing system, such as a mobiletelephone, a tablet, a notebook computer, a set-top box, or some othertype of computing system, the external connection 502 may be a networkconnection such as, but not limited to, any version of the followingprotocols: Institute of Electrical and Electronic Engineers (IEEE)802.3, IEEE 802.11, Data Over Cable Service Interface Specification(DOCSIS), digital television standards such as Digital VideoBroadcasting (DVB)-Terrestrial, DVB-Cable, and Advanced TelevisionCommittee Standard (ATSC), and mobile telephone communication protocolssuch as Global System for Mobile Communication (GSM), protocols based oncode division multiple access (CDMA) such as CDMA2000, and Long TermEvolution (LTE).

The memory device 510 may include an array 517 of memory cells. Thememory cells may be organized into using any architecture and may useany type of memory technology. Address lines and control lines 503 maybe received and decoded by control circuitry 511, I/O circuitry 513 andaddress circuitry 512 which may provide control to the memory array 517through the word line drivers 514 and/or the sense circuitry 515. I/Ocircuitry 513 may couple to the data lines 504 allowing data to bereceived from and sent to the supervisory circuitry 501. I_(OFF)compensation circuitry 516 may work in conjunction with the sensecircuitry 515 to correctly determine the state of memory cells of thememory array 517 by compensating for the off-current of the memory array517. Data read from the memory array 517 may be temporarily stored inread buffers 519.

The system illustrated in FIG. 5 has been simplified to facilitate abasic understanding of the features of the system. Many differentembodiments are possible including using a single supervisory circuitry501 to control a plurality of memory devices 510 to provide for morestorage space. Additional functions, such as a video graphics controllerdriving a display, and other devices for human oriented I/O may beincluded in some embodiments.

The flowchart and/or block diagrams in the figures help to illustratethe architecture, functionality, and operation of possibleimplementations of systems, methods and computer program products ofvarious embodiments. In this regard, each block in the flowchart orblock diagrams may represent a module, segment, or portion of code,which comprises one or more executable instructions for implementing thespecified logical function(s). It should also be noted that, in somealternative implementations, the functions noted in the block may occurout of the order noted in the figures. For example, two blocks shown insuccession may, in fact, be executed substantially concurrently, or theblocks may sometimes be executed in the reverse order, depending uponthe functionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

Examples of various embodiments are described in the followingparagraphs:

An example method to access a memory cell may include sensing anoff-current of a set of memory cells during a sample period, providing acompensation current at the sense node during an access period, anddetermining a state of a memory cell of the set of memory cells duringthe access period. In the example method the set of memory cells arecoupled to a sense node and the compensation current is dependent on theoff-current. An example method to access a memory cell may includedetermining an off-current of a set of memory cells that are coupled toa sense node, accessing a memory cell of the set of memory cells duringan access period, and compensating for the off-current of the set ofmemory cells to determine a state of the memory cell. In some examplemethods the determining of the off-current may include storing digitalinformation regarding measurements of the off-current during a sampleperiod, the accessing of the memory cell may include capturing a digitalvalue based on a measurement of current through the memory cell duringthe access period, and the compensating for the off-current may includesubtracting at least some of the digital information from the digitalvalue to determine a state of the memory cell. In some example methodsthe compensating for the off-current may include providing acompensation current at the sense node during the access period, and theaccessing of the memory cell may include sensing a voltage level at thesense node to determine a state of the memory cell. A maximum value ofthe compensation current may be based on the off-current. In someexample methods the determining of the off-current may includedeselecting the set of memory cells and providing a reference current atthe sense node during a sample period, and setting a circuit to provideup to the maximum value of the compensation current at the sense nodeduring the access period. In some example methods the accessing of thememory cell may include selecting the memory cell, and disabling asource of the reference current. In some example methods the maximumvalue of the compensation current, plus a value of the referencecurrent, plus a value of the off-current, is about equal to zero, thesample period occurs before the access period, and the value of thereference current is less than an expected on-current of the memorycell. In some example methods the determining of the off-current mayinclude deselecting the set of memory cells and providing a referencecurrent and a load current at the sense node during a sample period, andsetting a circuit to provide up to the maximum value of the compensationcurrent at the sense node during the access period. In some examplemethods the accessing of the memory cell may include selecting thememory cell, providing the load current to the sense node, and disablinga source of the reference current. In some example methods the maximumvalue of the compensation current, plus a maximum value of the loadcurrent, plus a value of the reference current, plus a value of theoff-current, is about equal to zero, the sample period occurs before theaccess period, and the value of the reference current is less than anexpected on-current of the memory cell. In some example methods the setof memory cells may include NAND strings of memory cells, stacks ofmemory cells, polysilicon channels, or phase change material. Someexample methods may also include accessing another memory cell of theset of memory cells during a second access period, and compensating forthe off-current during the second access period without re-determiningthe off-current. Some example methods may also include re-determiningthe off-current if more than a predetermined period of time has passedsince the determining of the off-current, accessing another memory cellof the set of memory cells during a second access period after there-determining of the off-current, and compensating for there-determined off-current during the second access period. In someexample methods the predetermined period of time is greater than about 1microsecond. In some example methods the determining of the off-currentoccurs in response to a change of temperature of the set of memorycells. In some example methods the accessing of the memory cell occursin response to a read command, a program command, or an erase command.Any combination of the examples of this paragraph may be used inembodiments.

An example memory may include two or more memory cells coupled to asense node, with the two or more memory cells having a combinedoff-current. The example memory may include sense circuitry, coupled tothe sense node, to determine a state of a memory cell of the two or morememory cells in response to an access of the memory, and compensationcircuitry to compensate for the combined off-current. In some examplememories, the two or more memory cells may include two or more NANDstrings of memory cells. In some example memories, the two or morememory cells may include one or more stacks of memory cells. In someexample memories, the two or more memory cells may include polysiliconchannels. In some example memories, the two or more memory cells mayinclude phase change material. In some example memories, thecompensation circuitry may include reference current circuitry andcompensation current circuitry coupled to the sense node. In someexample memories, may also include control circuitry to deselect the twoor more memory cells and enable the reference current circuitry toprovide a reference current at the sense node, during a sample period.The control circuitry may also set the compensation current circuitry,during the sample period, to provide up to a maximum value ofcompensation current at the sense node during an access period thatoccurs after the sample period, wherein the maximum value of thecompensation current is based on the combined off-current and thereference current. The control circuitry may also select the memory celland disable the reference current circuitry, during the access period,in response to the access of the memory. In some example memories, thereference current is less than an on-current of the memory cell, and avoltage level of the sense node during the access period indicates thestate of the memory cell. In some example memories, the maximum value ofthe compensation current, plus a value of the reference current, plus avalue of the combined off-current, is about equal to zero. Some examplememories may also include load current circuitry to provide a loadcurrent at the sense node. The maximum value of the compensationcurrent, plus a maximum value of the load current, plus a value of thereference current, plus a value of the combined off-current, may beabout equal to zero, in some examples. In some example memories, thesample period is initiated by the control circuitry in response to achange of temperature of the two or more memory cells. In some examplememories, the control circuitry, during a second access period initiatedin response to a second memory access, may be further capable to selectanother memory cell of the two or more memory cells, disable thereference current circuitry, and enable the compensation currentcircuitry to provide up to the maximum value of compensation current atthe sense node. In some example memories, the voltage level of the sensenode during the second access period indicates the state of the anothermemory cell. In some example memories, the control circuitry, if morethan a predetermined period of time has passed since the sample period,during a second sample period may be further capable to deselect the twoor more memory cells, enable the reference current circuitry to providethe reference current at the sense node, and re-set the compensationcurrent circuitry to provide up to a new maximum value of compensationcurrent at the sense node during the second access period, wherein thenew maximum value of the compensation current is based on the referencecurrent and a combined off-current of the two or more memory cellsduring the second sample period. The second sample period occurs beforethe second access period in some examples. In some example memories, thepredetermined period of time is greater than about 1 microsecond. Anycombination of the examples of this paragraph may be used inembodiments.

An example electronic system may include supervisory circuitry togenerate a memory access, and at least one memory coupled to thesupervisory circuitry. The at least one memory may be described in thepreceding paragraph. Some example electronic systems may also includeI/O circuitry, coupled to the supervisory circuitry, to communicate withan external device. Any combination of the examples of this paragraphand the preceding paragraph may be used in embodiments.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” include plural referents unless the contentclearly dictates otherwise. Furthermore, as used in this specificationand the appended claims, the term “or” is generally employed in itssense including “and/or” unless the content clearly dictates otherwise.As used herein, the term “coupled” includes direct and indirectconnections. Moreover, where first and second devices are coupled,intervening devices including active devices may be located therebetween.

The description of the various embodiments provided above isillustrative in nature and is not intended to limit this disclosure, itsapplication, or uses. Thus, different variations beyond those describedherein are intended to be within the scope of embodiments. Suchvariations are not to be regarded as a departure from the intended scopeof this disclosure. As such, the breadth and scope of the presentdisclosure should not be limited by the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and equivalents thereof.

What is claimed is:
 1. A method to access a memory cell, the methodcomprising: determining an off-current of a set of memory cells that arecoupled to a sense node; accessing a memory cell of the set of memorycells during an access period; and compensating for the off-current ofthe set of memory cells to determine a state of the memory cell.
 2. Themethod of claim 1, wherein the determining of the off-current comprisesstoring digital information regarding measurements of the off-currentduring a sample period; the accessing of the memory cell comprisescapturing a digital value based on a measurement of current through thememory cell during the access period; and the compensating for theoff-current comprises subtracting at least some of the digitalinformation from the digital value to determine a state of the memorycell.
 3. The method of claim 1, wherein the compensating for theoff-current comprises providing a compensation current at the sense nodeduring the access period; and the accessing of the memory cell comprisessensing a voltage level at the sense node to determine a state of thememory cell; wherein a maximum value of the compensation current isbased on the off-current.
 4. The method of claim 3, wherein thedetermining of the off-current comprises deselecting the set of memorycells and providing a reference current at the sense node during asample period, and setting a circuit to provide up to the maximum valueof the compensation current at the sense node during the access period;and the accessing of the memory cell comprises selecting the memorycell, and disabling a source of the reference current; wherein themaximum value of the compensation current, plus a value of the referencecurrent, plus a value of the off-current, is about equal to zero; thesample period occurs before the access period; and the value of thereference current is less than an expected on-current of the memorycell.
 5. The method of claim 3, wherein the determining of theoff-current comprises deselecting the set of memory cells and providinga reference current and a load current at the sense node during a sampleperiod, and setting a circuit to provide up to the maximum value of thecompensation current at the sense node during the access period; and theaccessing of the memory cell comprises selecting the memory cell,providing the load current to the sense node, and disabling a source ofthe reference current; wherein the maximum value of the compensationcurrent, plus a maximum value of the load current, plus a value of thereference current, plus a value of the off-current, is about equal tozero; the sample period occurs before the access period; and the valueof the reference current is less than an expected on-current of thememory cell.
 6. The method of claim 1, further comprising accessinganother memory cell of the set of memory cells during a second accessperiod, and compensating for the off-current during the second accessperiod without re-determining the off-current.
 7. The method of claim 1,further comprising: re-determining the off-current if more than apredetermined period of time has passed since the determining of theoff-current; accessing another memory cell of the set of memory cellsduring a second access period after the re-determining of theoff-current; and compensating for the re-determined off-current duringthe second access period.
 8. The method of claim 7, wherein thepredetermined period of time is greater than about 1 microsecond.
 9. Themethod of claim 1, wherein the determining of the off-current occurs inresponse to a change of temperature of the set of memory cells.
 10. Themethod of claim 1, wherein the accessing of the memory cell occurs inresponse to a read command, a program command, or an erase command. 11.A memory comprising: two or more memory cells coupled to a sense node,wherein the two or more memory cells have a combined off-current; sensecircuitry, coupled to the sense node, to determine a state of a memorycell of the two or more memory cells in response to an access of thememory; and compensation circuitry to compensate for the combinedoff-current.
 12. The memory of claim 11, wherein the two or more memorycells comprise two or more NAND strings of memory cells.
 13. The memoryof claim 11, wherein the two or more memory cells comprise one or morestacks of memory cells.
 14. The memory of claim 11, wherein the two ormore memory cells comprise polysilicon channels.
 15. The memory of claim11, wherein the two or more memory cells comprise phase change material.16. The memory of claim 11, wherein the compensation circuitry comprisesreference current circuitry and compensation current circuitry coupledto the sense node; and the memory further comprising control circuitryto: deselect the two or more memory cells and enable the referencecurrent circuitry to provide a reference current at the sense node,during a sample period; set the compensation current circuitry, duringthe sample period, to provide up to a maximum value of compensationcurrent at the sense node during an access period that occurs after thesample period, wherein the maximum value of the compensation current isbased on the combined off-current and the reference current; and selectthe memory cell and disable the reference current circuitry, during theaccess period, in response to the access of the memory; wherein thereference current is less than an on-current of the memory cell, and avoltage level of the sense node during the access period indicates thestate of the memory cell.
 17. The memory of claim 16, wherein themaximum value of the compensation current, plus a value of the referencecurrent, plus a value of the combined off-current, is about equal tozero.
 18. The memory of claim 16, further comprising load currentcircuitry to provide a load current at the sense node, wherein themaximum value of the compensation current, plus a maximum value of theload current, plus a value of the reference current, plus a value of thecombined off-current, is about equal to zero.
 19. The memory of claim16, wherein the sample period is initiated by the control circuitry inresponse to a change of temperature of the two or more memory cells. 20.The memory of claim 16, wherein the control circuitry, during a secondaccess period initiated in response to a second memory access, isfurther capable to select another memory cell of the two or more memorycells, disable the reference current circuitry, and enable thecompensation current circuitry to provide up to the maximum value ofcompensation current at the sense node; wherein the voltage level of thesense node during the second access period indicates the state of theanother memory cell.
 21. The memory of claim 20, wherein the controlcircuitry, if more than a predetermined period of time has passed sincethe sample period, during a second sample period is further capable to:deselect the two or more memory cells; enable the reference currentcircuitry to provide the reference current at the sense node; and re-setthe compensation current circuitry to provide up to a new maximum valueof compensation current at the sense node during the second accessperiod, wherein the new maximum value of the compensation current isbased on the reference current and a combined off-current of the two ormore memory cells during the second sample period; wherein the secondsample period occurs before the second access period.
 22. An electronicsystem comprising: supervisory circuitry to generate a memory access;and at least one memory coupled to the supervisory circuitry, the atleast one memory comprising: two or more memory cells coupled to a sensenode, wherein the two or more memory cells have a combined off-current;sense circuitry, coupled to the sense node, to determine a state of amemory cell of the two or more memory cells in response to the memoryaccess; and compensation circuitry to compensate for the combinedoff-current.
 23. The electronic system of claim 22, wherein the two ormore memory cells comprise at least one of NAND strings of memory cells,stacks of memory cells, polysilicon channels, or phase change material.24. The electronic system of claim 22, further comprising; I/Ocircuitry, coupled to the supervisory circuitry, to communicate with anexternal device.
 25. The electronic system of claim 22, wherein theelectronic system comprises a solid state drive.
 26. The electronicsystem of claim 22, wherein the compensation circuitry comprisesreference current circuitry and compensation current circuitry coupledto the sense node; and the at least one memory further comprises controlcircuitry to: deselect the two or more memory cells and enable thereference current circuitry to provide a reference current at the sensenode, during a sample period; set the compensation current circuitry,during the sample period, to provide up to a maximum value ofcompensation current at the sense node during an access period thatoccurs after the sample period, wherein the maximum value of thecompensation current is based on the combined off-current and thereference current; and select the memory cell and disable the referencecurrent circuitry, during the access period, in response to the memoryaccess; wherein the reference current is less than an on-current of thememory cell, and a voltage level of the sense node during the accessperiod indicates the state of the memory cell.
 27. The electronic systemof claim 25, wherein the maximum value of the compensation current, plusa value of the reference current, plus a value of the combinedoff-current, is about equal to zero.
 28. The electronic system of claim25, the at least one memory further comprising load current circuitry toprovide a load current at the sense node, wherein the maximum value ofthe compensation current, plus a maximum value of the load current, plusa value of the reference current, plus a value of the combinedoff-current, is about equal to zero.
 29. The electronic system of claim25, wherein the control circuitry, during a second access periodinitiated in response to a second memory access from the supervisorycircuitry, is further capable to select another memory cell of the twoor more memory cells, disable the reference current circuitry, andenable the compensation current circuitry to provide up to the maximumvalue of compensation current at the sense node; wherein the voltagelevel of the sense node during the second access period indicates thestate of the another memory cell.
 30. The electronic system of claim 29,wherein the control circuitry, if more than a predetermined period oftime has passed since the sample period, during a second sample periodis further capable to: deselect the two or more memory cells; enable thereference current circuitry to provide the reference current at thesense node; and re-set the compensation current circuitry to provide upto a new maximum value of compensation current at the sense node duringthe second access period, wherein the new maximum value of thecompensation current is based on the reference current and a combinedoff-current of the two or more memory cells during the second sampleperiod; wherein the second sample period occurs before the second accessperiod.